(1) Field of the Invention
The present invention relates to a hydraulically operated transmission having an electromagnetic proportional reduction valve mounted on a pressure oil supply line leading to a hydraulic change speed clutch, clutching pressure of the hydraulic clutch being controllable by controlling electric current applied to the reduction valve.
(2) Description of the Prior Art
Among known hydraulically operated transmissions is the type which, when operating the clutch, applies a control current or voltage that increases from a predetermined level with progress of time, to the electromagnetic proportional reduction valve in order to reduce shocks accompanying the clutch operation.
In this type of transmission, however, the control current or voltage increases monotonically. Because of this and a great build-up resistance of the electromagnetic proportional reduction valve, there is a delay in the internal pressure increase of the hydraulic clutch and hence a long time taken for clutching action.
In order to eliminate the above disadvantage and to shorten the time required for the clutch operation, attempts have been made to improve the control current to be applied to the electromagnetic proportional reduction valve as disclosed in Japanese Patent Publication Kokai No. 61-136044, for example. None of such attempts made heretofore are satisfactory from the point of view of optimal clutch control to be achieved where a considerable pressure loss occurs between the reduction valve and the hydraulic clutch and where a great load or a small load acts on the transmission.